This invention relates to an engine control system and, in particular, to a charge forming device to effect split engine operation of a closed loop controlled multi-cylinder internal combustion engine.
Closed loop or feed back control of a multi-cylinder internal combustion engine is known in which the quantity of fuel fed to the engine is controlled in response to the output from an oxygen sensor, which detects the oxygen concentration of the exhaust gases discharged from all cylinders of the engine, in order to maintain the air fuel ratio of the mixture to be supplied to the engine at stoichiometry to cause a three-way catalytic converter, in the engine exhaust system, to operate most efficiently.
It is also known to operate a multi-cylinder internal combustion engine by suspending the supply of fuel to selected cylinders, less than the total number, to render them inactive and by increasing the load on the remaining active cylinders to operate them efficiently under high load conditions to improve the fuel economy of the engine.
This is known as split engine operation. Applying this split engine operation to a closed loop controlled internal combustion engine provided with a three-way catalytic converter will cause the air fuel ratio of the exhaust gases to deviate considerably from stoichiometry toward the lean side when the engine is shifted into split engine operation mode because at this engine operation mode the exhaust gases discharged from the active cylinders will be diluted with air discharged from the remaining inactive cylnders.
As a result, the quantity of fuel to the active cylinders will be increased excessively in accordance with the output from the oxygen sensor which represents the oxygen concentration of the exhaust gases containing oxygen from the air discharged from the inactive cylinders, thereby to deteriorate the driveability, thus worsening the fuel economy of the engine.
In order to prevent this, it is known to maintain the intake and exhaust valves for the inactive cylinders in their closed conditions to prevent air induction and air discharge by these cylinders. With this, overenrichment of the air fuel mixture charge to the active cylinders will be prevented and the pumping loss by the inactive cylinders will be reduced. The problem encountered in this known system is that since the inactive cylinders operate with their intake and exhaust valves in closed conditions, the pistons are likely to suffer from compression loss.